In recent years, research and development are conducted on a nonvolatile memory device having memory cells that include variable resistance elements. A variable resistance element is an element that has a property of changing in resistance value (changing between a high resistance state and a low resistance state) according to an electrical signal and enables information to be written by this change in resistance value.
One structure of memory cells using variable resistance elements is a cross point structure. In the cross point structure, each memory cell is formed at a different one of cross points of orthogonally arranged bit lines and word lines so as to be positioned between a bit line and a word line. Various types of such cross point variable resistance nonvolatile memory devices are developed in recent years (for example, see Patent Literatures (PTLs) 1 to 4).
PTL 1 discloses a nonvolatile memory device having memory cells that use bidirectional variable resistors in the cross point structure. In this nonvolatile memory device, for example a varistor is used as a bidirectional nonlinear element included in each memory cell, in order to reduce a leakage current which flows into an unselected memory cell.
PTL 2 discloses a writing method in a nonvolatile semiconductor memory device having resistance memory elements each of which has a high resistance state and a low resistance state and changes between the high resistance state and the low resistance state by voltage application. In the writing method, when changing a resistance memory element from the low resistance state to the high resistance state, a predetermined constant voltage that enables a resistance change is applied to the resistance memory element, thereby changing the resistance memory element to the high resistance state. When changing the resistance memory element from the high resistance state to the low resistance state, a predetermined constant current that enables a resistance change is caused to flow through the resistance memory element, thereby changing the resistance memory element to a resistance state of a low resistance value corresponding to the value of the current.
PTL 3 describes a memory device that achieves higher integration. The memory device includes: a semiconductor substrate; a cross point memory cell array formed above the semiconductor substrate and having memory cells in a three-dimensional multilayer arrangement, each of the memory cells having a stack structure of a programmable resistance element and an access element, the programmable resistance element being written to a high resistance state or a low resistance state in a nonvolatile manner according to a polarity of an applied voltage, and the access element having a resistance value in an OFF state in a certain voltage range that is at least ten times as high as that in a selected state; and a read/write circuit formed on the semiconductor substrate so as to be situated below the memory cell array, for reading and writing data from and to the memory cell array.
However, PTL 3 does not disclose such a writing method as described in PTL 2 in which, when changing a programmable resistance element from the high resistance state to the low resistance state, a predetermined constant current that enables a resistance change is caused to flow through the programmable resistance element to thereby change the programmable resistance element to a resistance state of a low resistance value corresponding to the value of the current.
PTL 4 discloses, in a three-dimensional multilayer cross point variable resistance memory cell array, a hierarchical bit line structure realized in a small area and a layout method for such a structure. In the hierarchical bit line structure, short-segmented local bit lines are connected to global bit lines via switches for selectively switching the connection, in order to reduce a leakage current to an unselected memory cell and ensure a stable operation.